SCIENTIFIC PICTURES OF JOSEPH WRIGHT 437 



its luminous vapour furnishes the source of light which gives 

 the characteristic lighting effect. 



One might at first be inclined to think that the small hole 

 and the jet of luminous vapour were the product of the artist's 

 imagination, but reference to contemporary literature justifies 

 these features, and shows that Wright had carefully studied the 

 available descriptions of the process. The quotations which 

 follow are from a work of the period, which was translated 

 from the French at the time when Wright was at work on the 

 picture and which he may have consulted : 



" The retort ought to be well luted to a receiver of moderate 

 size, pierced with a small hole, and half full of water. For 

 this purpose ordinary fat lute may be bound on with strips 

 of linen, dipped in a lute prepared with lime and whites of 

 eggs. The hole in the furnace through which the neck of the 

 retort passes ought to be well stopped with furnace earth. 

 Lastly a small wall of bricks is raised between the furnace and 

 receiver, to guard this vessel against heat as much as possible." 



" The retort is to be heated by slow degrees during an hour 

 and a half ; and then the heat is to be increased till the retort 

 be red-hot and the phosphorus begins to pass in luminous 

 vapour : when the retort is almost of a white red heat the 

 phosphorus passes in drops, which fall and congeal in the water 

 at the bottom of the receiver." 



" Phosphorus is a kind of sulphur composed of a peculiar 

 acid united with phlogiston. This matter is extremely fusible, 

 as we have seen. It has, like sulphur, two kinds of inflamma- 

 tion ; one very weak, emitting a flame not powerful enough 

 to kindle other combustible matters, but sufficient for the 

 gradual consumption and burning of its own phlogiston ; 

 the other is vivid, brilHant, and strong, accompanied with 

 decrepitation, and capable of kindling instantly any combustible 

 matter. These two flames of phosphorus are easily distinguish- 

 able during the distillation of it, when the small hole in the 

 receiver is unstopped ; for when the vessels are not too much 

 heated, the flame which issues through the hole is luminous 

 in the dark, but does not kindle combustible matter ; it may 

 be touched without danger, and it only renders the hands 

 that touch it luminous. But when the vessels are too much 

 heated, the flame issues with more force ; it then decrepitates, 

 and is capable of burning any person that touches it. The 

 flame is a sign that the heat is too strong, and therefore when 

 it is perceived, the fire ought to be lessened." 



There are also instructions for boring the hole — which was 

 to be half a line in diameter and closed with a wooden peg — ■ 

 as well as for making lutes. 



